1. Field of the Invention
This invention relates generally to an optical system for coupling pump light into a signal fiber of a fiber array laser/amplifier and, more particularly, to an optical system including a tapered light guide through which pump light is coupled into a signal fiber of a high power, dual-clad fiber array laser/amplifier.
2. Discussion of the Related Art
High power lasers are employed for many applications, such as military applications against a variety of airborne threats, including ballistic missiles, cruise missiles and tactical aircraft. A diode-pumped, solid-state laser amplifier employing an array of fibers is one known high power laser used for these types of applications. U.S. Pat. No. 6,229,939, titled High Power Fiber Ribbon Laser and Amplifier, assigned to the assignee of this invention, and herein incorporated by reference, discloses such a high power fiber array laser amplifier.
A typical high power fiber array laser includes an array of spaced apart single-mode fibers each generating a separate laser beam that are combined into a single beam to provide the high power output. Fiber array lasers of this type may include a hundred or more single-mode fibers each generating about a hundred watts of power. Each fiber in the array typically includes a round single-mode core having a diameter on the order of 5-20 μm to generate the laser beam. An inner cladding layer formed around the core and having a lower index of refraction than the core traps the single-mode beam within the core. An outer cladding layer having a lower index of refraction than the inner cladding layer contains pump light within the fiber and directs it across the core to be absorbed therein and amplify the beam by a physical process well understood to those skilled in the art.
A single-mode laser beam generates the most power per unit area when the beam is focused. As the number of transverse modes of the laser beam increases, the size of the beam spot that can be focused also increases as a result beam diffraction. This reduces the beam power per unit area, which reduces its intensity.
The power output of a fiber laser can be increased by increasing the length of the core of the fibers and providing more optical pump light. However, the material of the core has power limits that if exceeded may damage the core material. Multiple single-mode fibers are thus required to generate the desired total beam output power. More optical power can also be provided by making the core diameter larger. However, as the core diameter increases, the generation of higher-order propagation modes begin to develop, and it becomes increasingly more difficult to limit the beam to a single-mode. Further, as the size of the core and the power increases, the generation of heat in the core also increases. Cooling systems can be employed to reduce the heat, but larger diameter cores make it more difficult to remove the heat from the center of the core. Therefore, a heat gradient may exist across the array, which causes a decrease in performance of the laser.
It is necessary to couple the proper amount of pump light into the signal fibers in the fiber array to amplify the signal and provide the desired beam power. Various techniques are known in the art to couple the pump light into the signal fibers. In one particular design, a diode emitter is coupled to one end of a pump light guide fiber and the opposite end of the guide fiber is butt coupled to an end of the signal fiber so that pump light from the diode emitter is coupled into the signal fiber. Because the diode emitter only generates a minimal amount of pump light, additional pump light needs to be coupled into the signal fiber to meet the power requirements of the fiber. Therefore, many pump light guide fibers, each being coupled to a separate diode emitter, are fused together and butt coupled to the end of each signal fiber in the array so that the light of many diode emitters is coupled into the signal fiber.
For example, a typical diode emitter may generate 1-2 watts of power. A typical single-mode fiber can handle more than 100 watts of power. Therefore, about 50-100 separate emitter diodes need to be provided for each signal fiber in the array, which may include up to 100 signal fibers. In some designs, it is not possible to couple that many emitters to each fiber, and thus power is sacrificed. Further, the emitter's brightness limits the fraction of the power that can be coupled into a fiber array of a fiber laser/amplifier. Because so many diode emitters are required for the entire fiber array, current commercial pump light sources are very expensive, for example, $40,000 for a 250 W unit
U.S. Pat. No. 6,243,515, titled Apparatus for Optically Pumping an Optical Fiber from the Side, issued Jun. 5, 2001 to Hefflinger, et al., describes an apparatus for optically pumping an optical signal fiber from the side. Diode pump light is first collimated by a lens, and then coupled transversely into a signal fiber using a reflective diffraction grating formed along a short section of the signal fiber. The diffracted pump light travels in the signal fiber and is contained therein by total internal reflection (TIR). The technique disclosed in the '515 patent for coupling pump light into a fiber requires a lens for each diode emitter. Further, additional diode emitters can only be positioned adjacent to each other at sufficiently long intervals to ensure efficient pump light absorption and to prevent deleterious light leakage at the next input location.